The tool, which was produced and sponsored by Optum, is our latest effort to help people identify and plan for costs they will face in retirement. It turns out that health care is a much, much larger expense than I had ever imagined — even for those with supplemental Medicare and retiree health benefits through their employer. In fact, according to the calculator, I should expect to spend about $311,000 — out of my own pocket — on health care during my 30-plus years of retirement. Most of the sticker shock comes from the cost of premiums, deductibles and coinsurance requirements. The tool also considers costs associated with specific diseases that run in my family (hello, cancer), and provides some tips on how to manage those diseases (hello, vegetables).

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Apparently, I'm not alone in my naïveté about retiree health care costs. In a recent AARP study, we found that almost two-thirds of respondents have never tried to figure out how much their health care will cost them in retirement. And when we asked them to give a ballpark estimate of how much money they might need, over 40 percent guessed that they would need less than $100,000 to cover their health costs throughout retirement.

Clearly, the tool fills an all-too-common gap in the retirement-planning process. Check it out as you make your retirement plans. In the meantime, here are a few things to keep in mind about health costs:

Medicare is composed of four parts: Part A (also known as "Original Medicare," which provides hospital care), Part B (nonhospital medical care), Part C (also known as "Medicare Advantage," private HMO or PPO plans that combine parts A, B and D, and may have lower out-of-pocket costs) and Part D (prescription drug coverage).

Although most people don't pay premiums for Part A (because they already paid for it through their payroll taxes), there are deductibles and coinsurance requirements for all four parts, and premiums to pay for parts B, C and D.

AARP Retirement Planning Tools

When you're choosing the best Medicare plan for you, the lowest premium may not be the lowest-cost plan overall. Even Part A charges an annual deductible of more than $1,100. And if you do end up hospitalized with just Part A coverage, let's hope you don't stay long: After two months in the hospital, you're expected to pay almost $300 per day out of your own pocket. After three months, you'll be charged almost $600 per day.

Out-of-pocket charges may be lower and more predictable under a Part C Medicare Advantage plan than under Original Medicare. The downside to these Advantage plans is that they usually limit your choice of doctors and hospitals.

You should plan to reevaluate your Medicare coverage needs each year during the open enrollment period, which runs from Oct. 15 to Dec. 7.

If you're under 65 and have a high-deductible insurance plan ($1,250 or more), you might want to consider setting up a health care savings account now. Your contributions are tax-deductible, they're invested in the stock market, and they grow tax-free. And if you use the money to pay for medical expenses, you're not taxed on the withdrawal. Best of all, unlike traditional flexible spending accounts, the money rolls over from year to year, helping you build a health care nest egg for your later years.

Bear in mind that the calculator doesn't compute the cost of long-term care. With any luck, you won't need it. But again, that's not something I would gamble on.

Jean C. Setzfand is vice president of the Financial Security issues team in the Education and Outreach group at AARP. She leads AARP's educational and outreach efforts aimed at helping Americans achieve financial peace of mind in retirement. She can be reached at jsetzfand@aarp.org and you can follow her on Twitter @JSetz.